Site testing at Dome C:

recent results

CONCORDIASTRO Project

E. Aristidi, A. Agabi, E. Fossat, T. Travouillon, M. Azouit, J. Vernin, A. Ziad, F. Martin, Sadibekova T.

www-luan.unice.fr/Concordia

South Pole 1979

Main characteristics of the site

1. Altitude > 3000 m2. Slope < 1/10003. Snow < 5g/cm/year4. Limit for auroras2

5. Limit of visibility fo geostationary satelites

Altitude level

Some reasons for expecting good observingconditions in astronomy are obvious

• Very cold => good for IR, and sub-mm• Very dry (0.15 - 0.20 mm H20 in winter) => also

good for the same ranges• High altitude (barometric=3700m) => good

transparency to be expected at all wavelength• long days and long nights => good for programs

requiring long integration• access to extreme southern stars during extended

durations

Wind

• winds appear to be slow throughout all atmosphere => good for image stability andscintillation

• Prevailing slow winds => good for mechanicalstructures and for astronomical image quality

ConcordiAstro site testing :3 experiments

Balloons : Step 1 : PTU

Step 2 : Cn2

DIMM/GSM

Step 1 : seeing

Step 2 : 0, L0, 0

To obtain a complete astronomical qualification of the site from the turbulence side

Goal :

Mast

Monitor the

ground layer Cn2

Vol 186

Wind Speed Profiles at Dome C (Dec 2000)

Wind Speed Profiles at Paranal ESO Chili

(1992)

Wind Speed Profiles at Gemini NOAO Chili

(1998)

Vol 45 Vol 118

Wind Speed Profiles

Alt

itud

e (K

m)

Alt

itud

e (K

m)

Alt

itud

e (K

m)

Concordiastro : 4 (+1) summer campaignsWho Stay Balloon Telescope

1995 J. Vernin 1 week 5 (+1) 0

2000-01A Agabi

JM Clausse1 week 6 (+1) 0

2001-02A Agabi

J Dubourg6 weeks 34 1

2002-03A Agabi

E Aristidi

T. Travouillon5 weeks 61 1

2003-04

A Agabi

E Aristidi

E Fossat

T. Travouillon3 months 96 2

The experiments

Estimating the seeing :Differential Image Motion Monitor

• Celestron 11 d=28 cm, f = 2.8 m, tube in INVAR

• 2 holes mask on pupil diam. D=6 cm sep. B=20 cm

• glass prism deviation=30  arcsec

• CCD max sensitivity=500 nm pixel size=10 microns thermostated at –20°C

Glass prism

Overall cost ~30 k€

DIMM Principle

The transverse (t2) and

longitudinal (l2) variances of

the spots position difference gives two estimates of the seeing .

Assuming Kolmogorov turbulence (infinite outer scale), we have (Tokovinin, 2002, PASP 114, 1156)

Estimating isoplanatic angle

Principle : scintillation measurement with a circular 10cm diameter pupil with 4 cm central obstruction

Ziad et al., 2000, Appl. Opt. 39, 30

BalloonsIn-situ soundings to obtain the turbulent energy profile Cn

2(h)

RS80 radiosond

Balloon

rB=33 cm

rA= 95 cm

Thermometers Send : TA , TB

P, T, U wind speed & direction

Measurement of TA2 ,

TB2

Calculation of

CT2=< TA

2 > rA-2/3

CT2=< TB

2 > rB-2/3

2 estimates of Cn2

Then…

Principle

(Borgnino et al., 1979, A&A 79, 184)

Inflating the Balloon

In summer

In winter

Preparing the sond…

Launching the balloon

In summer

In winter

Summer turbulence

conditions

November 2003: amazing days

! Values not corrected from z and exposure time (10 ms)

Wow ! Excellent !

Summer seeing : statistics

N data 31597 Std deviation 0.39

Mean seeing (arcsec)

0.66 Seeing max 5.22

Median seeing 0.54 Seeing min 0.08

3

0.54

(based on 2 summer campaigns)

Seeing as function of time

Good news for solar astronomy : seeing below 0.5 almost every day at tea time during ~6h

Good seeing when surface layer temperature gradient vanishes

(Aristidi et al., A&A 2005)

Temp. Gradient (6°/100m)

No temp. gradient

Isoplanatic angle: statistics

Maidanak 2.47 Ziad et al. 2000

Oukaimeden 1.58 Ziad et al. 2000

South Pole 3.23 Marks et al. 1999

Paranal 1.91 Ziad et al. 2000

La Silla 1.25 Ziad et al. 2000

Pachon 2.71 Ziad et al. 2000

N data 6328

Mean (arcsec) 6.8

Median 6.8

Std dev 2.4

Max 17.1

Min 0.7

6.8

6.8

Comparison with other sites

Site Seeing Isoplan. angle

Paranal 0.66 1.91

La Silla 0.87 1.25

Maidanak 0.70 2.47

South Pole 1.74 3.23

Dome C (summer)

0.54 6.8

The best site of the world ?

0

20

40

60

80

100

0.0 0.2 0.4 0.6 0.8 1.00

100

200

300

400

CP

MK

Cou

nts

Total Atmospheric Seeing (arcsec)

DC

Cum

ulat

ive

Pro

babi

lity

Night seeing at Dome C

SODAR + MASS

Travouillon et al

Towards the winter

summer seeing 0.54 arcsec

AASTINO results : 0.27 arcsec in autumn

We were very confident for the winter !

First winterover

10 Feb: Deparure of the last plane

Karim Agabi :The winterastronomer

Remote-controlling (useful at –70°C…)

Wi-Fi LAN+Fiber optics connection

Data acquisition Concordia labo

To the mast (700 m)

300 m

About the weather

Statistics 2005 : about 85 % 2006 : systematic, visual, measurements

about 80% in summer, 90 % in April

36 days 74 days

Autumn seeing

Some vertical profiles…

Everything is inthe surface layer !

Ground seeing: >1 arcsec

Seeing in altitude: <0.4 arcsec

How high is the surface layer ?

40 m 40 m

20 m

T=20°

Estimating turbulence parametersfrom balloon Cn

2(h) profiles

Seeing

Isoplanatic angle

Coherence time

Cn2(h)

h1

Parameters can be computed from Cn2(h) and the wind profile v(h)

Changing h1 : compute parameters that would be observed at alt. h1

wind speed

Surface layer

South Pole : 220m

Dome C : 30m

R.D. Marks, et al. 1999, A&A

Optical/interferometric parameters

Integrated from h=8m

Balloons

Seeing (arcs) 0.4

0 (ms) 11.2

0 (arcs) 5.3

Integrated from h =30m

Balloons (10)

Dimms (March- May 05)

Seeing (arcs) 1.6 1.2

0 (ms) 7.0

0 (arcs) 5.3 3.6

AASTINO 2004 data

0.27 ‘’

7.9

5.7 ‘’

s

Interferometric coherence times

= 0.31 r0/ v ~7 ms

= 0.31 L0/ v ~775 ms

L0 = 10 m

GSM h=3.5m

1 h > 30 m

3 h > 0 m

opd

Comparison with other sitesSite (arcs) (ms) Lo(m)

La Silla 0.9 1.3 1.5 25

Paranal 0.9 1.9 3.0 24

Pachon 0.9 2.7 3.0 28

Maidanak 0.7 2.5 6.6 28

Mauna Kea 0.8 2.9 2.4 18

San Pedro 0.7 2 1.2 27

South Pole 1.9 3.2

Dome C (0)

1.6

5.3 7 10

H> 30m 0.4

5.3 11.2

ECMWF (European Center for Medium range Weather Forecast)

http://www.ecmwf.int

• 60 pressure levels from surface 655mB to 0.1 60 pressure levels from surface 655mB to 0.1 mB mB

• 0h, 6h, 12h, 18h UT0h, 6h, 12h, 18h UT

• Parameters : Parameters :

pressure (mB), pressure (mB),

temperature (temperature (ooC), C),

relative humidity (%), relative humidity (%),

zonal et meridian wind speed projections (m/s)zonal et meridian wind speed projections (m/s)

• Two types of sondes, RS80 and RS90 (more precise on the humidity and temperature parameters)

• Examples of the comparison between ECMWF analysis and balloons measurements

RS80 ____ balloons data ____ model

RS90

Differences: Model – Data

RS80 – 168 used balloons

RS90 – 48 used balloons

Temperature rms 1.5 - 3 ºC for RS80

1 - 1.5 ºC for RS90

Relative humidity rms 1 - 10 % for RS80

1 - 2 % for RS90

Wind speed rms 1m/s at all altitudes (figure).

• Optical turbulence forecast ?

Turbulence = temperature gradient + wind

Turbulence above Dome C can be produced mostly at:

1. Tropause

2. Ground layer

Tropause (4-6km above ground):

- In summer inversion of the temperature gradient;

- No tropopause in winter!!!

Average monthly wind speed (m/s) at:     200mB     250mB      300mB_________________________________________

January   6.57     8.15       9.91 February   10.26   14.18     15.89 March       9.39   10.88     12.21 April       10.47   10.94     12.01 May        12.60   12.95     13.54 June       12.93   13.93     14.21 July       12.94   13.45     13.68 August     17.56   17.84     16.70 September   12.69   13.64     13.56 October   10.85   10.65     10.76 November   12.08   13.46     14.89 December     6.40     8.66   11.21

- The Coherence time of the wavefront is defined by (Roddier, 1981):

o ~ 1/Vo

where Vo is velocity of the turbulence

- And Sarazin&Tokovin (2001) proposed an expression for Vo which related to metrological variables only:

Vo = Max(0.4V200Mb)

Atmospheric turbulence modelH. Gallee, M. Swain

• Instantaneous (“snap shot”) profiles show strong and fast boundary layer seeing nearly always present over Antarctic ice sheets.

• Models predicts large improvement in seeing and coherence time above boundary layer.

• Model predicts Dome C has 1.16“ average seeing at 8 m elevation.

• Dome C boundary layer most probable elevation is ~22 m.

• Good agreement between model and observations for elevations below 1000 m.

For best results, place telescope above blue line

Auroras

• SSS

• Photometer (v)

• MOSP

• DIMM

• GSM

• Pistonscope

• Mast

Increase the statistics over more than one year

, 0,

Cn²(z), L0(z)

Cn²(z), V(z)

L0, 0,

Cn²(hi), up to 40m

opd, opd

2006 - 2007Instruments for the next winter

Extinction coefficient

Future instruments

• AIRBUS (Near IR sky brightness)

• IRAIT (80 cm IR telescope, general user)

• A-STEP (40 cm telescope 30’x30’ photometer)

• ICE-T (2x80 cm wide-field photometer)

• MYKERINOS (Prototype interferometer 3x40 cm)

•

Observability